1. Field of the Invention
The present invention relates in general to inkjet printheads, and more particularly to substrate heaters for heating the ink in the printheads of inkjet printers.
2. Description of the Related Art
The process of printing employing inkjet techniques requires a thermally controlled environment to maintain a desired print quality and color consistency. The thermal energy generated within the integrated circuit of a printhead heats the ink held therein. Ideally, the temperature of the ink should remain constant at a desired temperature. A change in the temperature of the ink results in the change in the properties of the ink, including the viscosity, surface tension, droplet size, etc. The print quality changes as these ink parameters change. Over very short periods of time, the ink is jetted from numerous nozzles many times. In order to cause jetting of ink from a printhead, the ink drops are ejected by a process of nucleating a single bubble at an intense heat for a very short duration. This process is repeated thousands of times per second for each nozzle. This results is an accumulation of heat that raises the temperature of the ink, which is undesirable. On the other hand, when the printer is idle for some period of time, the ink tends to cool without the use of some type of heater. In addition to the foregoing, all portions of the printhead are generally not at the same temperature. Rather, some areas of the printhead can be warmer or cooler than other areas of the printhead. These gradients in the printhead temperature can be dynamic, meaning that they change over time as a function of various reasons, including the pattern of nozzle use, ventilation, ambient temperature, etc. These variations in the temperature of the ink can lead to poor print quality that is visible. Thus, the management of the printhead temperature is not an easy task. Thermal control systems have been incorporated on inkjet printhead integrated circuits to sense temperature and apply heat as needed to maintain the ink at a constant temperature independent of print pattern density. The heating systems require transistor switching devices to turn on and off the heating elements. The switching devices and heating elements require some physical area in the printhead integrated circuit and contribute to the die size and ultimately to the printhead die cost.
Some integrated circuit heating systems, for example, the Non-Nucleating Heating (NNH) system and the printhead integrated circuit heating system disclosed in U.S. Pat. No. 7,384,115 by Barkley, use the same heater and switch device used by the nozzle jetting system. In this heating system, the nozzle heaters are addressed with a pulse energy that is sufficient to generate substrate heat, but insufficient to nucleate the ink and jet a droplet from the nozzle. The disadvantage of this technique is that it can only be used to generate substrate heat when the particular nozzle heater is not jetting. While these systems are attractive because they minimize silicon area, they require an additional pin to implement the short duration pulses required to prevent jetting during heating. The additional pin adds to die width and increases the cost of the printhead. In addition, using the same transistor switching device (power FET) that is used in the jetting system ages the switch and causes an unnecessary shift in key parameters over the life of the printhead.
Furthermore, these NNH systems require either switch matrices or multiplexers to control whether the heater is using the inkjet fire signal or the substrate heating signal. These multiplexers also require additional silicon area on the semiconductor substrate. Some inkjet printhead systems use heating elements around the periphery of the printhead integrated circuit and thus do not add to the die width because the heating elements are located in vacant spaces along the edges of the semiconductor die. These heating systems apply heat away from the nozzle jetting heaters and are not as effective because they are not located near the inkjet nozzles.
As noted above, when utilizing a temperature control system in an inkjet printhead, there must also be provisions for sensing the temperature, and through a feedback loop, controlling the temperature of the semiconductor substrate. Attempts have been made to place temperature sensors at various locations in the substrate, it being understood that the outer edges of the semiconductor substrate tend to be cooler as the thermal energy can be more easily dissipated to the air or to the structure to which the substrate is mounted. The temperature control of the substrate is efficient, but often the temperature sensors only sense the temperature at a particular location and serve to control the temperature as such location, while the nozzle structure locations still experience temperature gradients, albeit at a smaller degree. Some substrate heater designs tend to locate the heater systems at efficient peripheral locations on the substrate, while neglecting to consider that it is the nozzle locations that require precise temperature control.
U.S. Pat. No. 6,357,863 by Anderson et al., discloses a linear substrate heater for an ink jet printhead. Here, incorporated into the integrated circuit are resistive nozzle jetting heaters and substrate heating resistors. The substrate heating resistors are located closer to the edge of the silicon chip than to the ink reservoir. The substrate heating resistors are selected with different resistance values to accommodate the different amounts of heat generated at different areas of the semiconductor chip.
U.S. Pat. No. 6,102,515 by Edwards et al., discloses a printhead driver employing both nozzle jetting heaters and a substrate heater. The two substrate heaters are located at opposite ends of the semiconductor chip, outside the area where the jetting heaters are located. The jetting heaters and the substrate heater can be activated separately or together using enable signals and corresponding enabling circuitry, without the use of a separate driver for the substrate heater. U.S. Pat. No. 7,163,272 by Parish et al., discloses the use of additional nozzle jetting heaters for the purpose of heating the substrate, as opposed to the use of other nozzle jetting heaters for heating the ink to nucleate the same into a bubble.
It can be seen from the foregoing that various attempts have been made to incorporate heaters into the integrated circuit of a printhead. While exotic and complicated heating systems are an option to carefully control the substrate heat, and thus the temperature of the ink, such heating systems generally function well at the expense of using much more silicon area, which increases the cost of the printhead, and makes the printhead more prone to failure because of the complexity thereof.
From the foregoing, it can be seen that a need exists for a temperature control for an inkjet printhead that maintains the substrate areas adjacent the nozzle structures at a constant temperature, where temperature control is necessary. A need exists for distributing the substrate heating elements adjacent the nozzle structures to concentrate the thermal energy where it is necessary. Another need exists for a substrate heating system where both the heating elements and the switching transistors, which switch the heating element on and off, are co-located next to the corresponding nozzle structures. A further need exists for a substrate heating system that includes series-connected heating resistors distributed with the nozzle structures, and parallel-connected switching transistors, also distributed and located next to the nozzle structures with a heating resistor. Another need exists for a printhead that incorporates a substrate heater system therein while yet minimizing semiconductor area and requiring no additional pins or terminals.